1. Field of the Invention
This invention relates to an electric device using organic materials, which is applicable to a solar battery, a photoelectric conversion device and a transistor, and more particularly to an electric device comprising a portion in which an electrode, an n-type inorganic semiconductor layer, and an electron-conductive organic layer are successively overlaid in this order.
2. Discussion of Background
Electric devices using inorganic materials have conventionally been used for solar batteries, photoelectric conversion devices and transistors. In recent years, however, electric devices using organic materials have taken over the place of the conventional electric devices. This has been made possible by the recent improvement and diversification in characteristics of organic semiconductors, and the progress of thin-film forming techniques.
It is preferable that such electric devices have an electric current density of 1 mA/cm.sup.2 or more. In the case of a solar battery, for instance, a short-circuit photoelectric current density of 10 mA/cm.sup.2 or more is necessary in order to attain a conversion efficiency of 5% or more under sunlight. An organic electroluminescent device is also required to have an electric current density of approximately 10 mA/cm.sup.2 so as to obtain a luminance as high as 1000 cd/m.sup.2. Furthermore, a transistor capable of flowing a great amount of electric current can be utilized for various purposes applications.
As described above, it is an indispensable requirement that the electric device have a high electric current density from the viewpoint of practical use.
In order to attain a high electric current density, it is necessary that both positive holes and electrons can freely move inside the electric device. However, there are few organic materials which can fulfill this requirement In order to satisfy the requirement, it has been proposed that two organic layers, one serving as an electronconductive layer, and the other serving as a positive holeconductive layer, be separately formed.
However, even when the electric device comprises the above two layers, the electric current density decreases in the case where an energy barrier exists therein. The energy barrier is formed at the junction of any two layers made of different materials. Therefore, it is necessary that all junctions except those in a functional site be in ohmic contact which does not hinder the migration of electric charges. In the case of a photoelectric conversion device such as a solar battery, the site in which photoelectric charges are generated is the functional site.
The inventors of the present invention have carefully studied the above proposal, in which two organic layers serving as an electron-conductive layer and as a positive hole-conductive layer are separately formed in order to attain a high electric current density. As a result, problems were found as described below in the materials for a negative electrode.
In general, metals have conventionally been used as the a negative electrode materials. In particular, metals having a work function of less than 4.5 eV have been used so that the negative electrode can be in ohmic contact with an organic semiconductor layer.
For example, a photoelectric conversion device which has a maximum short-circuit electric current density of 3 mA/cm.sup.2 is disclosed in Japanese Patent Publication 62-4871. In this device, Ag and In are used as the materials for the negative electrode. It is generally known that these metals are readily oxidized.
With respect to an electroluminescent device, a device using polyvinyl carbazole as a luminescent material has been reported by R. H. Partridge in Polymer, 24, 748 (1983). In this device, Cs is used as the material for the negative electrode.
Furthermore, in Appl. Phys. Lett., 51, 913 (1987), C. W. Tang has reported an electric device composed of an aluminum quinoline complex layer and a diamine layer. In this device, the negative electrode is prepared by using an alloy of Mg and Ag, which is also easily oxidized.
It is considered that when metals having a work function of 4.5 eV or more are used as the materials for a negative electrode, the stability of the electrode can be enhanced, and the finished device can withstand practical use.
However, when such metals are used as they are for the electrode, the electric current density of the device is drastically decreased, and the characteristics of the device are deteriorated. This is because a barrier to electron conduction is formed between an electron-conductive organic semiconductor layer and an electrode made of a metal having a work function of 4.5 eV or more.
A photoelectric conversion device is a device which converts light applied thereto to electric energy. In general, a photoelectric conversion device using organic materials has a lower conversion efficiency than a device using inorganic materials. This is because organic materials have a low short-circuit electric current density J.sub.sc) and a small fill factor (ff).
A device having a conversion efficiency of 5% is required to have a short-circuit electric current density (J.sub.sc) of at least 10 mA/cm.sup.2 when a white light with an intensity of 75 mW/cm.sup.2 is applied thereto. However, as will be described later, a known solar battery using organic materials has a short-circuit electric current density which is much lower than the above value, and has an insufficient value of the fill factor (ff). One of the reasons for the low fill factor, is that the quantum efficiency of organic materials drastically decreases in a weak electric field. In order to overcome this problem, the device is required to generate a strong inner electric field. Furthermore, in order to obtain a large fill factor, it is also required that generated electric charges can reach the electrodes without being hindered by an electrical barrier.
Conventional electric devices are classified into three types as described below, depending on the fabrication method thereof.
(1) Electric devices containing a Schottky junction or a metal insulator semiconductor (MIS) junction:
These devices utilize an inner electric field generated at a junction of a metal and a semiconductor layer. Organic materials such as merocyanine dyes and phthalocyanine pigments have been reported as the materials for the semiconductor layer for use in this type device.
An electric device composed of Al, a merocyanine dye and Ag shows a conversion efficiency of 0.7% when a white light with an intensity of 78 mW/cm.sup.2 is applied thereto, as reported by A. K. Ghosh, et al. in J. Appl. Phys., 49, 5982 (1978). Other data regarding this device are as follows:
Open-circuit electromotive force (V.sub.oc)=1.2 V,
Short-circuit current density (J.sub.sc)=1.8 mA/cm.sup.2, and
Fill factor (ff)=0.25.
Only a p-type organic semiconductor can show a high conversion efficiency, so that metals having a low work function of less than 4.5 eV, such as Al, In and Mg, are employed. These metals are readily oxidized to form an MIS-type junction.
The open-circuit electromotive force (V.sub.oc) of the device of this type is relatively high. However, since metals having a work function of less than 4.5 eV, such as Al, In and Mg, are used as the materials for the electrode, the electrode has the disadvantages of low transparency and low stability due to oxidation. Practically, the transparency of the electrode is, in general, approximately 10%, and at most approximately 30%. Thus, it cannot be expected that the device of this type can have a high conversion efficiency.
(2) Electric devices containing a hetero p/n junction of an n-type lnorganic semiconductor layer and a p-type organic semiconductor layer:
These devices utilize an electric field which is generated when the n-type and p-type semiconductor layers are in contact.
CdS and ZnO are used as the materials for the n-type inorganic semiconductor. Merocyanine dyes and phthalocyanine pigments (Pc) have been reported as the materials for the p-type organic semiconductor.
An electric device composed of ITO, CdS deposited onto the ITO layer, ClAlClPc and Au shows a conversion efficiency of 0.22% when an AM-2 light with an intensity of 75 mW/cm.sup.2 is applied thereto, as reported by A. Horr, et al. in Appl. Phys. Lett, 42, 15 (1983). Other data regarding this device are as follows: V.sub.oc =0.69 V, J.sub.sc =0.89 mA/cm.sup.2, and ff=0.29.
An electric device composed of ITO, ZnO, a merocyanine dye and Ag shows a conversion efficiency of approximately 0.5% when a white light with an intensity of 70 mW/cm.sup.2 is applied thereto, as reported by K. Kudo, et al. in J. Appl. Phys., 19, L683 (1980). Other data regarding this device are as follows: V.sub.oc =0.4 V, J.sub.sc =1.1 mA/cm.sup.2, and ff=0.3-0.4.
In the-device of this type, electric charges are mainly generated in the organic layer which is generally made of a single organic material, so that the generation of electric charges is affected by the spectral sensitivity of the organic layer. No organic semiconductors have been found until now which are capable of strongly absorbing light in a wide wavelength range of 400 nm to 800 nm.
Therefore, the device of this type can overcome the shortcomings of low transparency and low stability of the device of the above type (1). However, the device is restricted by the spectral sensitivity of the organic material employed, and cannot show a high conversion efficiency.
(3) Electric devices containing a hetero junction of organic layers:
The devices utilize an electric field which is generated when (i) an organic material of n-type or having high electron affinity and (ii) a p-type organic material are in contact.
Dyes such as Malachite Green, Methyl Violet and a pyrylium compound, and condensed polycyclic compounds such as flavanthrone and perylene pigments have been reported as specific examples of the former organic material.
As the latter organic material, phthalocyanine (Pc) pigments and merocyanine dyes have been reported.
An electric device composed of ITO, CuPc, a perylene pigment and Ag shows a conversion efficiency of 0.95% when an AM-2 light with an intensity of 75 mW/cm.sup.2 is applied thereto, as reported by C. Tang in Appl. Phys. Lett, 48, 183 (1986). Other data regarding this device are as follows: V.sub.oc =0.45 V, J.sub.sc =2.3 mA/cm.sup.2, and ff=0.65.
With respect to the electric devices containing organic materials, the above value of the conversion efficiency is maximum so far. C. Tang also discloses, in Japanese Patent Publication 62-4871, a device having the same structure as that of the above device but using a perylene pigment of a different type. The conversion efficiency of this device is 1% (V.sub.oc =0.44 V, J.sub.sc =3.0 mA/cm.sup.2, ff=0.6).
The device of this type is most preferable among the devices described above since light can be applied to the transparent electrodes. Moreover, photoelectric charges can be generated in two organic layers, so that the range of spectral sensitivity can be broadened.
In fact, in view of the spectral sensitivities reported by Tang in the Appl. Phys. Lett., it is considered that electric charges are generated in the perylene pigment layer when light with a wavelength of 450 to 550 nm is applied to the device, but in the CuPc layer, electric charges are generated when light of 550 to 700 nm is applied. The device according to Tang shows a higher fill factor (ff) than those of other devices. From this fact it is considered that in Tang's device, a stronger inner electric field is generated than those generated in other devices. The device according to Tang, however, still has the shortcomings described below.
(i) Since the organic layer is as thin as 300 to 500 .ANG., (which thickness is particularly described as a preferable thickness in Tang's article), it is liable to suffer from a problem of pin holes. According to the experiment carried out by the inventors of the present invention, the probability of the occurrence of the short-circuit between two electrodes due to pin holes is relatively high.
The area of the electrode of the Tang's device is only 0.1 cm.sup.2 when an area of 1 cm.sup.2 or more is necessary for practical use. Thus the production yield of a larger electrode will become a problem in practical use.
(ii) According to Tang, it is required that the electrode be in ohmic contact with each organic layer. He has reported that the value of V.sub.oc decreases when the device is composed of ITO, a perylene pigment, CuPc and Ag which are overlaid in this order. It is considered that this is because the ITO layer is not in ohmic contact with the perylene pigment layer, and the CuPc layer is not in ohmic contact with the Ag layer.
In order to attain this ohmic contact, a metal which is in contact with an electron-accepting organic material is required to be stable. Such a metal has a low work function, and In, Ag, Sn and Al are disclosed as such metals. However, these metals are readily oxidized.